Insert, on duplicate update in PostgreSQL?

Question

Several months ago I learned from an answer on Stack Overflow how to perform multiple updates at once in MySQL using the following syntax:

INSERT INTO table          


        

Answer 1

Edit: This does not work as expected. Unlike the accepted answer, this produces unique key violations when two processes repeatedly call upsert_foo concurrently.

Eureka! I figured out a way to do it in one query: use UPDATE ... RETURNING to test if any rows were affected:

CREATE TABLE foo (k INT PRIMARY KEY, v TEXT);

CREATE FUNCTION update_foo(k INT, v TEXT)
RETURNS SETOF INT AS $$
    UPDATE foo SET v = $2 WHERE k = $1 RETURNING $1
$$ LANGUAGE sql;

CREATE FUNCTION upsert_foo(k INT, v TEXT)
RETURNS VOID AS $$
    INSERT INTO foo
        SELECT $1, $2
        WHERE NOT EXISTS (SELECT update_foo($1, $2))
$$ LANGUAGE sql;

The UPDATE has to be done in a separate procedure because, unfortunately, this is a syntax error:

... WHERE NOT EXISTS (UPDATE ...)

Now it works as desired:

SELECT upsert_foo(1, 'hi');
SELECT upsert_foo(1, 'bye');
SELECT upsert_foo(3, 'hi');
SELECT upsert_foo(3, 'bye');